The present invention relates to a flow heater for heating the liquid in a closed cycle in a motor vehicle according to the preamble of claim 1.
Besides emitting fuel heaters there are emission-free alternatives available for heating the interior of motor vehicles. These are particularly interesting for the installation in "no emission" electric motor vehicles. Among emission-free heating concepts are a heat pump, which uses the heat generated by the compression of cooling agents, or a PTC-air heating.
DE 44 36 791 A1 discloses a PTC-air heating for the use in motor vehicles, in which the air flows past heated corrugates ribs and is heated thereby. The corrugated ribs serve as heat exchangers and are heated by a warm liquid and/or by PTC-heating elements.
An alternative is the exclusive heating of the heat exchanger with heated cooling liquid from a cooling cycle provided in the motor vehicle. However, often the temperature of the cooling liquid heated by the cooling of the vehicle motor is not sufficient for heating the air flowing through the heat exchanger in a sufficiently rapid manner. Therefore, an additional heating of the cooling liquid is required before it flows through the heat exchanger. To this end, the cooling liquid is heated in an upstream flow heater by directing the cooling liquid through radiators which are heated by means of electrical heating elements.
In the known flow heaters of this type the heating elements are provides as resistor heating elements. They have extremely high temperatures at the heating source (spiral wound filament, heating spiral) which may cause an overheating of adjacent parts, if the temperature of the heating source is not controlled or monitored. To avoid thermal damages caused through the temperatures of these heating sources that are significantly above the temperature of the liquid to be heated the heating source and the radiators to be heated by the same are arranged with a distance to each other. However, this causes heat losses and increased dimensions for the flow heater.